1. Field of Invention
This invention relates generally to a bolt or screw-type threaded fastener having a drive head engageable by the driver head of a torque-producing tool, and more particularly to a security fastener having a tamper-proof drive head that is engageable only by a tool having a complementary driver head.
2. Status of Prior Art
The conventional fastener of the bolt or screw-type has a drive head that is so slotted that it can be driven by an ordinary screwdriver. Or the drive head may include a cruciform recess so that it can be driven by a Phillips driver.
With fasteners having conventional slotted drive heads, there is a marked tendency to cam out or to deform the head, mainly because of the relatively insecure coupling between the drive head of the fastener and the driver tool. Damage to the drive head is more likely to occur as the torque applied by the tool increases.
Moreover, many different tools may readily be fitted into a conventional slotted type of fastener drive head. Hence such fasteners are not resistant to unauthorized removal. While a Phillips arrangement offers somewhat greater security, the general availability of Phillips-type driver tools renders the Phillips arrangement unsuitable for tamper-proof or tamper-resistant applications. And where the head of the fastener has a large profile, it may be possible to remove the fastener by engaging the head with the parallel jaws of a gripping tool.
The prior art discloses various forms of tamper-proof fasteners which require special driver tools.
Thus the Bisbing et al. Pat. No. 4,258,596 shows a tamper-resistant fastener whose drive head is provided with three female sockets, at least one of which is eccentrically disposed with respect to the other two. This fastener may be installed or removed only by means of a special driver tool having a male head adapted to be received in the female sockets of the fastener head to permit turning of the fastener upon application of torque to the driver tool.
There are many situations, such as in toilet stalls, where it is essential that fastener heads be maintained in a clean and sanitary condition, and yet be resistant to unauthorized removal. Because of the recessed female sockets in the head of the Bisbing et al. fastener, this head tends to collect contaminants and is not easy to clean.
The Goldhaber Pat. No. 4,018,111 discloses a tamper-resistant threaded fastener whose drive head has a bore therein which is eccentrically disposed relative to the longitudinal axis of the shank of the fastener. The drive tool for this fastener has a key which enters the bore of the fastener head. This provides a relatively weak coupling between the tool and the fastener head.
The Herkes et al. Pat. No. 4,125,051 discloses a tamper-proof fastener having a low profile head designed to resist unauthorized removal, the upper surface of the head being configured to include a short cylindrical boss and ribs radially extending from this boss.
In the Simone et al. Pat. No. 4,171,662, a security screw is disclosed whose head is shaped to prevent its removal except by the use of a matching socket wrench.
The Rodseth Pat. No. 4,430,035 shows a ribbed drive head for a fastener and a mating driver tool having a driver head or socket which complements the fastener drive head for closely interfitting engagement therewith.
Of particular background interest is the Mekler Pat. No. 4,938,108, issued in 1990, for this patent contains a detailed account of prior U.S. and foreign patents which disclose various expedients to render threaded fasteners tamper-proof or theft-resistant.